Machines have been used to screen material for many years. The word xe2x80x9cmaterialxe2x80x9d is necessarily general, because screening apparatus are used in applications from wastewater treatment and food manufacturing to the processing of aggregates and metals. Regardless of the type of material to be screened, though, separation of material based upon relative size is a valuable undertaking.
Since the predominant consideration in screening material relates to the sizes of particles to be separated, machines have been devised to screen material on an increasing scale and with greater efficiency. In the most generic sense, a screening machine consists of a hopper for receiving material and a screen at or near the lower portion of the hopper. The screen normally will have a number of openings therein for allowing material particles up to a certain size to pass through the screen. Particles that are too large to pass through any screen opening are moved out of the hopper using a method such as gravitational direction of the larger particles toward an open end of the hopper.
In order to screen with greatest efficiency, most screening machines are designed to oscillate rapidly to encourage a phenomenon called xe2x80x9cstratification.xe2x80x9d When a container of various-sized materials is shaken, the smaller particles tend to move toward the bottom of the container, while the larger ones remain toward the top of the container. If a hole of a size roughly equivalent to the size of the smaller particles is made in the bottom of the container, the smaller particles will likely pass through the hole during shaking, having become xe2x80x9cstratifiedxe2x80x9d on the bottom of the container. In this manner, stratification increases the efficiency of the separation process.
Modern screening machines are typically rectangular boxes, usually sloped, with one or more levels of screen surface with which the material interacts. Each level of screen surface is supported by a frame to which the screen surface is fastened, bolted, or otherwise secured in order to support the screen surface against the load of material that is placed into the rectangular box and shaken. Although the screening machine may be horizontally-oriented, many screening machines are inclined at an angle between 15 and 30 degrees, with the material being fed into the upper end of the machine and being allowed to flow to the bottom end of the rectangular box, which is usually open-ended. In this manner, material which fails to pass through the screen may be collected and possibly re-screened.
In a screen machine such as the above, rapidly vibrating the screening machine as material is being processed in the machine actually causes the material to assume a fluid-like characteristic as it moves across the screen surface, which enhances the stratification properties of the machine. A common range of rates for vibrating the screen machine is 600 to 3600 RPM. The vibration of the machine is achieved by various methods, the most common of which involves having one or more weighted shafts connected or integral to the machine that, when rotated, throws the material in the machine away from the screen surface, allowing the smaller particles to come in contact with the screen surface and sift through.
Originally, screen media was made of woven wire cloth or perforated steel plate. Such media wears out rapidly when handling abrasive particles, so more wear-resistant materials such as rubber or polyurethane have been used to significantly reduce the frequency and cost of maintenance on screens. Panels made of these materials, though, will stretch until they break, so cable or fabric reinforcement has been molded into such screens to strengthen them. The reinforcement is tensioned between the side walls of the screen machine, thereby providing beneficial additional structural strength to the wear materials. Examples of such designs are seen in U.S. Pat. Nos. 4,819,809 and 4,857,176. Significantly, however, both of those patents do not innovate to the extent of the subject matter herein.
U.S. Pat. No. 4,819,809 teaches a vibratory screen having a flexible molded polyurethane body having screen openings therein, and many aramid fibers reinforcing the panel, with the entire screen being able to be tensioned in place on a vibratory screening machine. The patented screen is coated with polyurethane before tensioning of the screen, whereas the screen panel of the invention is tensioned before the panel is coated with resilient coating, resulting in greater versatility for the screen panel of the invention. Moreover, the screen panel of the instant invention involves a selection of materials that is broader than that contemplated by the patent.
U.S. Pat. No. 4,857,176 teaches a substantially similar vibratory screen to that of U.S. Pat. No. 4,819,809. However, the vibratory screen of U.S. Pat. No. 4,857,176 teaches a vibratory screen for use with an arched-bed screening machine, and as noted elsewhere herein, the screen panel of the invention is made to be installed on a flat-bed screening machine, a more efficient type of screening machine due to the fact that material being screened does not tend to work its way down the slope to the sides of the machine. Arched-bed screening machines of this type thus fail to maximize the screening area, because the material being screened moves away from the top of the arch, minimizing use of that part of the screen, and gravitates toward the lower portions of the screen, where a greater percentage of the load ensures that some material that would otherwise be screened is blocked by other material and fails to find a passage through the screen.
The screen surface of a typical screening machine is made of one or more removable screen panels attached to the frame in any of a number of ways. The screen surface may be arched in the center, being supported by an arched support frame. In this configuration, screen panels are stretched from wall to wall across the screen so that as the edges of a panel are drawn outward, the center of the panel is pulled down against the crest of the arch. The tighter the screen surface is drawn against the supporting frame, the more the panel or panels are kept from flogging, prolonging the life of the screen.
In known screening machines, the arch in the screen support frame is desirable to properly tension the panels, but the arch has an unfortunate effect of causing material riding on the screen surface to move toward the side walls of the machine, leaving the crest of the arch without material on it, thereby effectively reducing usable screen area. To address this and other deficiencies, several manufacturers introduced screen media systems incorporating a flat support frame (i.e., a xe2x80x9cflat-bedxe2x80x9d machine) with a number of smaller screen media modules mechanically fastened to it. These systems provide for easier maintenance and panel change-outs compared with tensioned systems, and they allow screen operators to fine-tune the separation they are achieving. For example, different panels on the same flat-bed machine can have various-sized openings, screening at different granularities on the same screening machine.
An important shortcoming of modular screen panels in known flat bed systems is that the panels are not tensioned. Instead, each screen module has rigid internal structural elements that assist in keeping the module anchored to the support frame, and also aid the screen bed to resist sagging or breaking under the load of material being screened. However, structural elements within the screen modules can significantly reduce the area that can be perforated, thereby reducing the efficiency of the screening machine. Furthermore, in portions of the module that lack structural elements, the load must be supported solely by the rubber or urethane. The result is an overly thick perforated section, with large bar widths between openings, and a correspondingly low number of openings in the panels.
The design deficiencies of flat-bed modular screen panels are magnified when trying to screen fine materials. The thickness of the screen reduces its resiliency, thereby inhibiting stratification of the material load being carried as fine particles have greater difficulty in settling through the bed. Reduced resiliency also allows material, especially damp material, to stick to the screen surface, blocking the openings in the screen. Even if such materials find an opening, small opening size (perhaps as small as 0.25 millimeters in diameter) and great thickness of the panel form a tunnel through which the materials must travel. This structural arrangement slows material separation, possibly blocking other material particles trying to move through the opening, or potentially causing particles to obstruct and permanently plug the hole. Clogging of tunnels is further exacerbated where the materials being screened are damp or where particles have a tendency to adhere to one another. For purposes of this application, xe2x80x9cdampxe2x80x9d materials will have approximate water content of 5 to 35 percent.
Because of these problems, modular screen panels for flat-bed screening machines have not been accepted for fine screening applications or in applications where high open areas are required for adequate screen capacity. Most screen operators would prefer the wear life, maintenance ease, and flexibility advantages afforded by modular screening systems in achieving a desired separation. However, an inability to achieve sufficient open area and a thin enough membrane force them to use wire cloth, and suffer the short wear life and high operating costs resulting therefrom.
What is needed, then, is a modular screen panel that enables the screening of materials on a flat bed or other screening machines, where such screen panels have tensioned fiber-reinforced screens. It is further needed to provide a screen panel having an increased use life, regardless of the type or granularity of materials to be screened.
The present invention generally relates to a screen panel configuration for use in modular material screening systems. More specifically, the present invention relates to a modular screen panel for use in flat-bed screening systems to dramatically increase the number of openings (and granularity of screening enabled thereby) that can be formed in the screen panel.
The screen panel of the invention incorporates a tensioned fiber support system, encased in a non-tensioned outer wearing cover made of polyurethane or rubber. In practice, the fiber tension which supports the material load on the screen is entirely internal to each modular panel. Each panel is fastened mechanically into an existing flat-bed modular screen support frame.
The internal tensioning frame consists of a load-bearing frame member along two opposite sides of the panel. A piece of threaded rod perpendicularly abuts each end of each load-bearing member, and the two opposed rods on each side are connected by and substantially enclosed in a tensioning collar resembling a very long nut. The threaded rod and collar thus act as a turnbuckle or jacking mechanism along each of the two opposite sides of the frame.
While the frame is in an untensioned state, fiber reinforcement is attached to the load-bearing members with pre-determined gaps between each strand of the fiber. The collars are then turned to cause the load-bearing members to move apart, thereby applying tension to the fiber. Tightening each side to a predetermined torque ensures uniform tensioning across the panel.
It is also possible to make a simpler tensioning mechanism by welding spring steel between the load-bearing members and introducing a compressive force on the spring steel to impart a slight bow to the panel. At that point, the fiber would be wound onto the panel, and upon releasing the compressive force, the spring steel would put the fiber into tension. Alternatively, the fiber may be pretensioned using an external tensioning mechanism, then placed on a fixed frame in the tensioned state.
After appropriate tensioning, the tensioned fiber is placed into a mold that is precisely machined to allow the fibers to be totally encased within the outer coating. Specifically, any fiber within the coating will be centered horizontally and near the lower surface in the vertical plane, such that there is a greater amount of polyurethane coating above the fiber than below the fiber. The coating will effectively separate openings in the screen panel. After the wear-resistant polyurethane or rubber coating is molded around the frame and concurrently around the fiber, a finished panel will result.
The invention includes a rectangular polyurethane or rubber screen panel with fiber reinforcement, where the fiber is held in tension by a frame mechanism that is internal to the screen panel. Openings in the screen panel are formed by polyurethane or rubber bars, with each bar that is oriented in a longitudinal direction having an internal fiber reinforcement, while each bar that is transverse to the longitudinal bars has no fiber reinforcement. The panel is held in a modular screen support frame by mechanical means such that the fiber reinforcement is held in tension, while the panel itself is not.
The use of high tensile strength fiber considerably facilitates smaller bars between openings than is possible with non-reinforced modular screen panels. This configuration allows more openings in the screen panel, and it also allows a thinner screen surface that eliminates the tunnels that are common in fine screens. Furthermore, the small width of the bars between openings allows for a very resilient screen surface that promotes better stratification of the material on the bed. The end result is a more efficient screen medium that can be used in a much broader range of applications than previous technology allowed.
Accordingly, it is an object of the present invention to provide an improved modular screen design for a material separator device.
It is a further object of the invention to provide a screen panel design utilizing polymer body reinforced by tensioned reinforcing members, but wherein the body itself is not placed in tension.
It is a further object of the invention to provide a modular screen panel yielding more efficient screening of various sized materials.
It is a further object of the invention to provide a modular screen panel for use in flat bed screening machines that is more resilient at higher loads than previous screen panels that are not reinforced.
In addition to the foregoing, further, objects, features, and advantages of the present invention should become more readily apparent to those skilled in the art upon a reading of the following detailed description in conjunction with the drawings, wherein there are shown and described illustrated embodiments of the invention.